The field of the invention is aircraft propulsion and power systems and operation.
Propelled aircraft such as airplanes and rotorcraft have traditionally used combustible fuels such as gasoline or diesel fuel in internal combustion engines or turbines/jets for propulsion. This dependence on such fuels is likely principally due to their extremely high energy content. Unfortunately, the use of such aircraft is not always desirable for a number of reasons including relatively higher costs for production, maintenance and training, a relatively high risk of failure during operation, and the noise and emissions (particularly CO2).
An alternative to the use of combustion engines and turbines/jets for propulsion is the use of high efficiency electric motors. Advanced, high efficiency electric motors and controllers have already proven their usefulness on a host of unmanned solar-powered aircraft like the AeroViroment Pathfinder, Centurian, and the recent 14 motor Helios. Unfortunately, solar power is not practical for general aviation, due to the large surface area required, the altitudes required to get over clouds (and the lower atmosphere), and the limitation of flying only during bright sunny daytime hours. Moreover, solar powered aircraft are very limited in speed and weight capacity and structurally unsafe for manned usage.
Attempts at using batteries to power an aircraft have also occurred. As an example, several attempts have been made to utilize rechargeable batteries in electrically powered aircraft such as the European Silent Ael and the Antares self launching gliders. In such gliders, the batteries are used for takeoff power to launch the glider to sufficient height to pick up a thermal and continue xe2x80x9cglidingxe2x80x9d, typically less than 8 minutes per charge. However, the weight of sufficient batteries for takeoff and any reasonable flight leaves no weight allowance for the pilot and passengers, thus rendering the airplane useless for typical piloted flight.
Unfortunately, when compared to the energy content of gasoline, most rechargeable batteries offer less than 3% of the specific energy per pound of gasoline. Even after considering the poor conversion efficiency of internal combustion engines of less than 25% (versus over 90% efficiency for electric motors), gasoline still has nearly a 10 to 1 advantage of specific energy and energy density over rechargeable batteries. Although the energy density of batteries has improved dramatically over the last 10 years, it still needs dramatic improvement in specific energy performance (and cost reduction) to become commercially viable and competitive with gasoline for practical electric vehicle use (particularly including aircraft). Recent developments in advanced battery performance, particularly with rechargeable NiMH, Li-Ion, and Lithium Polymer chemistries begin to close the gap on the energy density of gasoline, but are still insufficient to operate manned electric airplanes, and cost prohibitive for other aircraft applications.
The use of fuel cells for providing electrical energy are known, most existing fuel cells are not suitable for use in aircraft. Fuel cells are currently being studied for automotive use as possibly providing higher net energy densities than batteries. Unfortunately, many fuel cell systems used in automobiles are unsuitable for use in aircraft, primarily due to the weight and power drain of all the special components required for operation of such fuel cells. Such components typically include compressors and hydrators needed to condition the air, oxygen, and/or hydrogen for input into the fuel cell, as well as complex (and heavy) heat exchangers and cooling systems needed to get rid of the excess waste heat being produced by the fuel cell. The storage of the critical fuel, hydrogen, poses even more problems, particularly from a weight and safety standpoint. Use of reformers to strip hydrogen from traditional hydrogen rich fuels like gasoline, methanol, diesel fuel, etc. are being explored for automotive use, but add even further weight and complexity for aviation use, particularly on smaller aircraft. In one instance a regenerative fuel cell system was incorporated into an unmanned aircraft as described in U.S. Pat. No. 5,810,284. However, regenerative fuel cells utilize a closed cycle and therefore require that sufficient fuel for the fuel cells be stored on board. To do so, tanks sized to contain enough fuel for the maximum duration of flight must be included even if a particular flight is to be of a shorter duration. As an empty tank represents over 90% of the fuel storage weight, this is a significant weight penalty. A similar penalty is paid for storing the fuel byproduct before it can be converted back to a usable form. Moreover, such a system requires the use of an electrolyzer to convert the water byproduct to a form suitable for reuse by the fuel cell. Such an electrolyzer also adds significant weight to the aircraft.
Therefore, there is still a need to provide methods and apparatus for light weight, high efficiency, reliable, and safe methods of powering aircraft, which also create little or no emissions, and are quieter and easier to service than conventional hydrocarbon fuel consuming engines, particularly internal combustion engines.
The present invention is directed to electrically powered aircraft having fuel cells as at least a partial source of electrical energy. In many instances the electrical energy powers an electric motor used to propel the aircraft. In some instances, the electric output from the fuel cell would be augmented by power from special high power xe2x80x9csurgexe2x80x9d batteries for critical takeoff and climbing, where the maximum electric power is required. In preferred embodiments, such fuel cell powered aircraft will supply oxygen to the fuel cell either from a container of oxygen carried on board the aircraft, or from a ram scoop which directs air through which the aircraft is moving to the fuel cell.
It is contemplated that fuel cell powered aircraft as described herein will be suitable for both manned and unmanned applications, will be simpler to build, repair and operate, will provide improved safety and reliability, will generate very little noise and virtually no pollutants, and ultimately will have lower total life cycle costs than existing aircraft.
It is also contemplated that fuel cell powered aircraft as described herein will be suitable for generally aviation as they will meet one or more of the following requirements: if the aircraft is a fixed wing aircraft, it will have a wingspan of less than 200 feet; the aircraft will be capable of climbing at a rate of at least 1000 feet per minute; the aircraft will be capable of achieving speeds of at least 100 miles per hour; and/or the aircraft will be able to carry at least 2 people, including the pilot.
It is further contemplated that the use of a hydrogen generator as a source of hydrogen for a fuel cell used in an aircraft will reduce or eliminate the need to hydrate the hydrogen in order to protect the fuel cell resulting in a corresponding weight reduction in the fuel cell systems by allowing removal of or reduction in size of hydration components.
It is further contemplated that the use of a ram input air duct or a container of pressurized air/oxygen will eliminate the need for a compressor to provide pressurized oxygen to the fuel cell with a corresponding weight reduction in the fuel cell systems resulting from not including any such compressor.
It is still further contemplated that fuel cell systems can be practically used on aircraft if such systems are weight optimized through the use of one or more of the following: graphite end plates, titanium tie bars, light weight heat exchangers, carbon composite tanks, and carbon fuel manifolds.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components.